Diplacusis, also known as diplacusis binauralis or interaural pitch difference (IPD) is the perception of a single auditory stimulus as two separate sounds which may differ in pitch or in time. It is typically experienced as a result of sensorineural hearing loss. Onset is usually spontaneous and can occur at the time of an acoustic trauma or in the midst of an ear infection. Sufferers may experience the effect permanently, or it may resolve on its own. As with hearing loss and its other associated symptoms, there is currently no treatment available for the condition.

Diplacusis of Pure Tones

For people with unilateral cochlear hearing losses or asymmetrical hearing losses, the same tone presented alternately to the two ears may be perceived as having different pitches in the two ears. This is the phenomenon of diplacusis.
The magnitude of the shift can be measured by getting the subject to adjust the frequency of a tone in one ear until its pitch matches that of the tone in the other ear.

If a 1000 Hz pure tone is presented in both of a listener's ears, then they perceive one pitch. However, alternate the presentation between ears (i.e. 1000 Hz left, 1000 Hz right, 1000 Hz left, ...), and a given person will consistently mismatch these sinusoids the same amount between trials if doing a pitch-matching task.

Biological Explanation via Theories of Pitch of Pure Tones

• Place Theory
  • Looking for the edge of the wave for the pitch
  • Could explain diplacusis -- (small) differences between the two cochlea
• Temporal Theory
  • Looking at the phase locking to tell what the pitch is
  • Hard to explain diplacusis with this
  • There are some examples of pitch which don’t have an "edge" on the basilar membrane, which this would account for—i.e. white noise, clicks, etc

Effects of Sensorineural Hearing Loss (SNHL)

If one ear has normal thresholds and one has SNHL (a unilateral loss), there is a lot of diplacusis. As much as 15 – 20% (for example 200 Hz one ear => 800 Hz in the other). Hard to match because the SNHL ear hears it “fuzzily”.

Bilateral SNHL gives less diplacusis, but there are probably pitch distortions because both ears are “messed up” the same way. This would make problems with music and speech understanding.